1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst with nitrogen oxide (NO.sub.x) reducing properties that are little affected by exposure to exhaust gases.
2. Description of Related Art
Three-way catalysts have been widely used as engine exhaust gas purifying catalysts to simultaneously oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NO.sub.x). Such a three-way catalyst is known to have strong nitrogen oxide (NO.sub.x) purifying effects at air-fuel ratios of near 14.7 (the ideal air-fuel ratio). However, because there are high concentrations of oxygen in exhaust gases from what are known as "lean-burn" engines, which have recently been developed and made practicable in response to engine fuel regulations, these three-way catalysts can not effectively reduce or eliminate nitrogen oxides (NO.sub.x).
Zeolite catalysts, which are produced by depositing an active catalytic material, such as copper (Cu), on silicate carriers, show great promise as catalysts capable of purifying emissions or removing pollutants by decomposition of nitrogen oxides (NO.sub.x) in exhaust gas, in an atmosphere with a high concentration of oxygen, into nitrogen (N.sub.2) and oxygen (O.sub.2). Thus, investigations of numerous possible active materials have been conducted to achieve a zeolite catalyst with a high ability to reduce or eliminate nitrogen oxides (NO.sub.x) despite numerous aspects of exhaust gases. For example, a technique for reducing nitrogen oxides (NO.sub.x) with high efficiency in an atmosphere with a high concentration of oxygen has been proposed in Japanese Unexamined Patent Publication No. 3-202157. This technique attempts to achieve a catalyst containing or carrying copper (Cu), one or more alkaline earth metals, and one or more rare-earth elemental metals deposited on zeolite that has a good ability to reduce nitrogen oxides (NO.sub.x) at temperatures of up to 800.degree. C. and the durability required for extended periods of use.
However, although copper (Cu) ion-exchanged zeolite catalysts are considered effective in the reduction of nitrogen oxides (NO.sub.x) and generally display 90% or greater reduction rates for nitrogen oxides (NO.sub.x) in the laboratory, these reduction rates unavoidably decrease in an atmosphere with a high oxygen concentration associated with actual driving of practical lean-burn engines. This is caused by the altered chemical properties of exhaust gases discharged from practical automobiles and the like. Moreover, copper (Cu) ion-exchanged zeolite catalysts are deactivated at temperatures of 700.degree. C. and above, sometimes even undergoing structural decomposition. In addition, with breakaway of aluminum (Al), the copper (Cu) ion-exchanged zeolite catalysts tend to cause an activity deterioration or reduction when vapor or steam is present in the exhaust gas.
Thus, although numerous attempts have been made to improve or increase the thermal resistance of this type of catalyst, applicational problems remain in that the temperature range of catalytic activity to exhaust gases shifts toward the side of higher temperatures when the catalysts are heated at high temperatures and also shifts toward the side of lower temperatures in vapor-containing atmospheres.
As described in the above-mentioned publication, improved thermal resistance is observed for catalysts formed by depositing copper (Cu), alkaline earth metals, and rare-earth elemental metals on silicate carriers. However, shifts in the active temperature range due to heat have not been taken into consideration for these catalysts, and the rate of the reduction of nitrogen oxides (NO.sub.x) is poor at low temperatures below 300.degree. C.
During repeated starting and stopping of vehicles on which lean-burn engines are mounted, the temperature of engine exhaust gas drops at the inlet of a catalytic converter. Accordingly, catalysts are required to have stable properties with improved purification ability at low temperatures and high thermal resistance.
Accordingly, extensive research was conducted by the inventors into catalysts, formed by depositing noble metals on silicate carriers, that could be expected to have good nitrogen oxide (NO.sub.x) reducing properties in a range of low temperatures. As a result of this research, in spite of poor nitrogen oxide (NO.sub.x) reducing properties of iridium (Ir) alone as compared to those of platinum (Pt), catalysts which indispensably or essentially include noble metals, in particular platinum (Pt) and iridium (Ir), deposited on silicate carriers as active materials, were found to have excellent activity due to synergistic effects and, hence, good nitrogen oxide (NO.sub.x) reducing properties. Moreover, when rhodium (Rh) was used as an active material in addition to platinum (Pt) and iridium (Ir), the resulting catalyst was found to show improved heat resistance in addition to excellent activity in a range of low temperatures.
Catalysts of silicate carriers on which noble metals are deposited as active materials are good at reducing nitrogen oxides (NO.sub.x) in exhaust gas at low temperatures. However, the range of active temperatures, including temperatures at which these catalysts exhibit their maximal activity, is narrow, and heating of these catalysts in a steam-containing atmosphere causes a shift in the maximum active temperature. Accordingly, a drawback is present in that such effects prevent sufficient exploitation of the essential properties of these catalysts.
Further extensive research was conducted by the present inventors into metals which exhibit behaviors much like iridium (Ir) which, when it is deposited on silicate carriers and coexists with platinum (Pt), promotes catalytic activity. This research led to an innovative exhaust gas purifying catalyst exhibiting excellent exhaust gas purification activity, even in a range of low temperatures, and having improved low-temperature catalytic activity. This catalyst has one or more metals, selected from iridium (Ir), III-B group elements, and IV-B group elements, and platinum (Pt) deposited on silicates. This catalyst is disclosed in Japanese Patent Application No. 4-141508, filed on Jun. 2, 1992.
With the catalyst of the above-mentioned application including a silicate carrier with platinum (Pt) and iridium (Ir), which are noble metals, indium (In), which is one of the III-B group elements, and tin (Sn), which is one of the IV-B group elements, deposited thereon, a shift in activity temperature range, including the temperature of maximum activity after a heating process, was found to be small compared to that of conventional nitrogen oxides (NO.sub.x) reducing catalysts, and to display extremely little change in properties due to aging, in addition to the increased low-temperature activity.